Foundations are one of the most important aspects of construction. The foundation is the part of the structure which interacts with the earth, and when properly constructed, allows construction of buildings that will withstand the powerful forces of nature, such as gravity, soil swelling, frost heaving and hydrostatic pressure.
Footings are the structural members that transmit the concentrated loads of the structure to the soil. These members are formed in various shapes and sizes and are generally constructed of steel-reinforced concrete. The footings are usually a minimum of two to three times wider than the width of the foundation wall. The thickness of the footing is a function of the weight of the structure above and the strength of the soil below the footing. A thicker footing will be stronger than a thinner footing. The footing is usually installed immediately after excavation. The foundation is then constructed on top of the footing. Generally, the footing is constructed independently of the foundation, and is normally constructed from reinforced concrete cast directly into an excavation formed in the soil to penetrate through the zone of frost movement and/or to obtain additional bearing capacity. Foundations are also structural members, transmitting loads from buildings and other structures to the earth. Foundations are designed based on the load characteristics of the structure and the properties of the soils and/or bedrock at the site.
In general, the primary considerations for foundation support are bearing capacity, settlement, and ground movement beneath the foundations. Bearing capacity is the ability of the site soils to support the loads imposed by buildings or structures. Settlement occurs under all foundations in all soil conditions, although lightly loaded structures or rock sites may experience negligible settlements. For heavier structures or softer sites, both overall settlement relative to unimproved areas or neighboring buildings and differential settlement under a single structure can be a matter of concern. Of particular interest is settlement that occurs over time, as immediate settlement can usually be compensated for during construction. Ground movement beneath a structure's foundations can occur due to shrinkage or swelling of expansive soils due to climactic changes, frost expansion of soil, melting of permafrost, slope instability or other causes.
Many building codes specify basic foundation design parameters for simple conditions, frequently varying by jurisdiction, but such design techniques are normally limited to certain types of construction and certain types of sites, and are frequently very conservative. In areas of shallow bedrock, most foundations may bear directly on bedrock. In other areas, the soil may provide sufficient strength for the support of structures. In areas of deeper bedrock with soft overlying soils, deep foundations are used to support structures directly on the bedrock. In areas where bedrock is not economically available, stiff “bearing layers” are used to support deep foundations instead.
Generally, a construction project begins with a site investigation of soil and bedrock on and below an area of interest to determine their engineering properties including how they will interact with, on or in a proposed construction. Site investigations are needed to gain an understanding of the area in or on which the construction will take place.
The engineering properties of soils are affected by four main factors: the predominant size of the mineral particles; the type of mineral particles; the grain size distribution; and the relative quantities of mineral, water and air present in the soil matrix. To obtain information about the soil conditions below the surface, some form of subsurface exploration, such as obtaining a sample of the underlying soil, is required.
Soil samples are obtained in either “disturbed” or “undisturbed” condition. A disturbed sample is one in which the structure of the soil has been changed sufficiently that tests of structural properties of the soil will not be representative of in situ conditions, and only properties of the soil grains can be accurately determined. An undisturbed sample is one where the condition of the soil in the sample is close enough to the conditions of the soil in situ to allow tests of structural properties of the soil to be used to approximate the properties of the soil in situ.
Soil samples may be gathered using a variety of samplers. Some provide only disturbed samples, while others can provide relatively undisturbed samples. Samples can be obtained by methods as simple as digging out soil from the site using a shovel. Samples taken this way are disturbed samples. More sophisticated sampling methods can be used to obtain undisturbed samples.
During construction projects, it often becomes important, or may even be required, to monitor in situ conditions of the subsoil while the project is ongoing. Obtaining undisturbed representative samples can be difficult, if not impossible, in areas where concrete footings have already been set in place. There is a need, therefore, for a concrete footing system that will permit in situ sampling and encapsulate the metal reinforcement structure therein to protect it from corrosion. Thus, a form for a concrete footing solving the aforementioned problems is desired.